Various types of steel, rock and concrete armor blocks have been placed on the bottom of the sea, either as they are or as assembled artificial reefs; to grow seaweed and other marine plant necessary to provide feeding and living grounds for fish and shellfish. Also, various contrivances have been used with these structures, to form appropriately rugged surfaces on them.
However, their functions are limited to providing substrata for seaweed and other marine plants to put their roots down on.
Seaweed and other marine plants including phytoplankton grow on various dissolved nutrients in seawater, including, nitrogen, phosphorus, silicon, manganese and iron. Particularly ferrous ions of iron dissolved in seawater are said to make significant contributions to their growth.
With this growth promoting effect in mind, the inventors proposed, as per Japanese Provisional Patent Publication No. 335330 of 1994 (Japanese Patent No. 2577319), algae rearing materials, consisting of vitreous materials, containing ferrous ions embedded therein that releas; when sunk into the sea, ferrous ions, stably, over long periods of time. The algae rearing materials, just mentioned comprise, by weight, 30 to 70 percent silicon dioxide, 10 to 50 percent sodium oxide and/or potassium oxide, 5-50 percent iron oxide, plus manganese oxide and phosphorus pentoxide as required, and containing not less than 1 percent ferrous ions. They are, for example, coated on to the surface of appropriate structures which are sunk into the sea. They promote the growth of seaweeds and other marine plants by providing suitable rearing sites for long periods of time by continuously releasing minute traces of ions into the surrounding seawater.
Recently, however, the vitreous materials are required to release more ferrous ions with a smaller glass supplies to fulfill the following needs:
1) To vitalize the entirety of seaweed beds with very small quantities of growth promoting materials. PA1 2) To promote the growth of diatoms that become the initial feed in the rearing of abalone and sea-urchin by using smaller quantities of growth promoting materials releasing ferrous ions. PA1 3) To promote growth of phytoplankton in offshore areas where plankton is difficult to grow because of the shortage of iron, by spraying small quantities of growth promoting materials over wide areas. PA1 (1) An algae growing material consisting of a vitreous material capable of releasing ferrous ions into water and consisting essentially of, by weight, 15 to 50 percent SiO.sub.2, 1 to 35 percent either or both of Na.sub.2 O and K.sub.2 O, 30 to 70 percent B.sub.2 O.sub.3, and 1 to 30 percent either or both of FeO and Fe.sub.2 O.sub.3. PA1 (2) An artificial reef for growing algae consisting of a reef structure consisting of, at least in part, or covered with a vitreous material capable of releasing ferrous ions into water and consisting essentially of, by weight, 15 to 50 percent SiO.sub.2, 1 to 35 percent either or both of Na.sub.2 O and K.sub.2 O, 30 to 70 percent B.sub.2 O.sub.3, and 1 to 30 percent either or both of FeO and Fe.sub.2 O.sub.3. PA1 (3) A porous, algae growing material consisting of a porous, vitreous material having independent foamed pores, containing 1 to 40 percent either or both of FeO and Fe.sub.2 O.sub.3 and capable of releasing ferrous ions into water. PA1 (4) A porous, algae growing material consisting of a foamed vitreous material described in (3) above containing, in addition to the constituent described in (3), 15 to 50 percent SiO.sub.2, 1 to 35 percent either or both of Na.sub.2 O and K.sub.2 O, 30 to 70 percent B.sub.2 O.sub.3.
Conventional growth promoting materials, when added in large quantities to seawater in water tanks, or other closed systems, have a tendency to raise the pH of the seawater through the release of potassium, sodium and other alkalis. Acid additions are required to avoid this rise in pH. Therefore, growth promoting materials causing less pH increase are required.
Meanwhile, global warming caused by rising levels of atmospheric carbon dioxide presents a significant problem. Strategies on global warming that will lead to upsetting of ecosystem balances, and the rising of sea levels are being studied on a global scale.
One of possible solutions attracting attention is the growing of marine algae. While seaweed and seagrass grow on the bottom of relatively shallow coastal sea not deeper than 20 m, phytoplankton is distributed in large areas of sea all over the world to depths where sunlight penetrates. It is known that these marine plants annually absorb approximately 30 billion tons of carbon from seawater and convert it into organic matter. If, as such, the number and species of marine plants, especially phytoplankton that make up the greater part thereof, are increased, they will absorb and fix more carbon dioxide from seawater. If the carbon dioxide in seawater decreases, the sea will make up for the loss by taking in an equivalent amount of carbon dioxide from the atmosphere because of the equilibrium relationship between the atmosphere and the sea, thereby decreasing the amount of carbon dioxide in the atmosphere.
Based on this already known relationship, preliminary experimentation has already been carried out in the growing of phytoplankton by spraying solutions of ferrous ions over sea surface areas containing sufficient amounts of nitrogen, phosphorus, silicon and other elements needed by phytoplankton but lacking iron required for their growth. Such experimentation has proved that solutions of ferrous ions spread over the surface of the sea are effective for growing phytoplankton (see Martin et al. (1994); Testing the Iron Hypothesis in Ecosystems of the Equatorial Pacific Ocean, NATURE, vol. 378(8), Sept. pp. 123-129).
Although their effectiveness has been thus confirmed, spreading solutions of ferrous ions requires large quantities of water and enormous costs for transportation. Besides, it is difficult to maintain the effect of ferrous ions for long periods of time because their solutions readily diffuse from the area in which they are sprayed. This problem may be solved by supplying iron compounds that release ferrous ions at and near the surface of the sea. However, ordinary iron compounds and metallic iron cannot provide a long-lasting phytoplankton growing effect because they quickly sink from the surface to the bottom by virtue of their high specific gravities. Also, ordinary iron compounds and metallic iron are unsuitable for practical use as they cannot continue the stable release of ferrous ions over long periods of time.
While some vitreous materials, releasing ferrous ions of have been already proposed as mentioned earlier, it is desired to use such vitreous materials that release ferrous ions more efficiently with smaller glass supplies.
Now, an object of this invention is to provide algae growing materials consisting of vitreous materials, having greater capabilities to release ferrous ions.
Another object of this invention is to provide artificial reefs or other forms of seaweed beds prepared by shaping said algae growing materials, into various shapes or incorporating them in various substrata structures, that are to be placed in artificial rearing sites or facilities or in natural sea areas.
Algae, especially phytoplankton, grow by photosynthesis while floating or drifting at or near the surface of the sea or at depths having adequate light penetration. Ferrous ions promote the photosynthesis of phytoplankton. Therefore, it is necessary to continue a stable supply of ferrous ions to depths with adequate light penetration, preferably to depths of a few meters. Thus, still another object of this invention is to provide porous, algae growing materials capable of a long-lasting stable supply of ferrous ions, in sea areas where phytoplankton is distributed and consisting of vitreous materials releasing ferrous ions and having lower specific gravities than seawater.